Default value for variable in C++ using cin >>

Question

I have written this code in C++ in the CodeBlocks IDE but when I run it, it doesn't give me -1 if it doesn't read a number, it gives me 0. Is there something wrong with the code?

#include "iostream"

using namespace std;

int main()
{
    cout<<"Please enter your first name and age:\n";
    string first_name="???"; //string variable
                            //("???" means "don't know the name")
    int age=-1; //integer variable (-1 means "don't know the age")
    cin>>first_name>>age; //read a string followed by an integer
    cout<<"Hello, " <<first_name<<" (age "<<age<<")\n";

    return 0;
}

Answer 1

The behaviour of std::basic_istream::operator>> has changed from C++11. Since C++11,

If extraction fails, zero is written to value and failbit is set. If extraction results in the value too large or too small to fit in value, std::numeric_limits::max() or std::numeric_limits::min() is written and failbit flag is set.

Note that until C++11,

If extraction fails (e.g. if a letter was entered where a digit is expected), value is left unmodified and failbit is set.

You could check the result by std::basic_ios::fail or std::basic_ios::operator! and set the default value by yourself. Such as,

string first_name;
if (!(cin>>first_name)) {
    first_name = "???";
    cin.clear(); //Reset stream state after failure
}

int age;
if (!(cin>>age)) {
    age = -1;
    cin.clear(); //Reset stream state after failure
}

cout<<"Hello, " <<first_name<<" (age "<<age<<")\n";

See also: Resetting the State of a Stream